How much setback from the boundary for a house on a slope? (BW)

And you would make the parapet, let's say, 10cm high in this case and run it down along the slope? How would you do it at the gable ends? Pull the parapet up to the highest point?

What do you mean by gable ends, and which highest point? I would make whatever one wants to call a parapet here follow the slope all around and have no idea about the dimensions, which depend on the specific construction.

Does one have to drain a flat roof "internally," i.e. on the roof surface, or could one also work with a rain gutter here?

Flat roofs of older construction are often (also in terms of negative slopes) drained internally, and yes, I would install gutters on the eaves sides here (seen from outside "behind" the parapet, so "internally," as stated, without overhangs). I would also derive my assessment of competence from this as to which construction the planner and contractor propose here.

How can I imagine such an "inner gutter"? In my drawn sketch, you would see a slight gable roof from the outside (street side). Would you have imagined it like this or raised the parapet so that one can guess a flat roof from the street side?

And so the more typical "very flat" gable roof with rain gutters visible on the eave sides would be.

How can I imagine such an "inner gutter"?

I gladly leave the detailed construction to your planner and executing contractor.

In my drawn sketch, one would see a slight gable roof from the outside (street side).

I think on the street side the higher gable (?) is located, accordingly the pitch slopes away from there, and you only see the gable.

Would you have imagined it that way or raised the parapet so that one could guess a flat roof from the street side?

I see the parapet only edging the substrate, the grass root, and about a thumb’s width above. From the street, one only sees the gable’s pitch at all. I would not raise anything higher than necessary, if only because of height restrictions / impact on setback areas.

Look at post #28, those are probably the minimum requirements. A green roof requires at least 30 cm plus drainage. Therefore, my suggestion is at least 40 cm. Without a parapet, it won't work. The slopes I suggested are easy to construct from a carpenter's perspective. You then won't need gutters, only 2 drains shown in red. Such a house with a 5-degree pitched gable roof looks like a shack. I hope your architect has better ideas.

Regarding the balcony, I would ask the city. If it is a subordinate component, it would probably protrude 1.5m beyond the building envelope.

Such questions are usually not answered by the building authority. A licensed planner must know something like that. In my opinion, your balcony is not a subordinate component.